"Flip-chip" manufacturing techniques involve soldering one or more semiconductor (silicon) chips (one is discussed), in face-to-face relationship, to another semiconductor chip termed a "substrate". Typically, solder balls (otherwise known as pads or bumps) are formed (raised above the planar surface of the chip and substrate) on facing surfaces of both the chip and the substrate at intended points of contact between the two, liquid flux (rosin) is often applied to the face of the chip and/or substrate, the chip is mechanically held in register with the substrate, and the chip and the substrate are subjected to elevated temperature to effect soldering, or fusion of the solder balls on the chip and the corresponding solder balls on the substrate.
The "solder balls" on either the chip or substrate, typically those on the substrate, may be solderable metallized surfaces. The soldering process may be carried out in a reducing atmosphere. A typical flip-chip structure is shown in FIG. 1, and is discussed in greater detail hereinafter.
Previous systems of rigid attachment of chips to chucks have been used for chip alignment, but they must allow some degree of compliance because the chips tend to change relative alignment during soldering by surface tension between the solder balls. The addition of liquid flux to the chip/substrate (flip-chip) assembly creates capillary attraction between the chip and the substrate, which serves to misalign the chip with respect to the substrate. This is illustrated in FIG. 2, and is discussed in greater detail hereinafter. Further, much of the flux that is applied to the flip-chip assembly is wasted. Still further, the dimension of the remaining gap between the chip and the substrate and the mechanical properties of the solder joints formed by the solder balls and corresponding solder balls tends to be indeterminate.